A swing arm type pick mechanism is used in some printers to feed sheets of paper or other print media into the printer. In a swing arm type pick mechanism, the feed roller is mounted to the end of a swing arm that pivots or “swings” above the media input tray. The pick roller rests on top of the stack of media sheets in the tray. A biasing spring applies a small force urging the swing arm down to keep the pick roller in contact with the top of the stack as sheets are used and the stack gets smaller. The swing arm pivot is located upstream (rearward) from the pick roller so that the pick roller will pull itself into the stack as it picks the top sheet and feeds it into the printer, rather than push itself away from the stack. The swing arm provides the force pressing down against the stack to create friction between the pick roller and the top sheet in the stack. The friction helps the pick roller grip the top sheet and move it off the stack. This pressing force is often called a “normal” force because the direction of the force is normal (perpendicular) to the stack. The rotating pick roller, by contrast, provides a force directed along the top sheet to move the sheet forward off the stack.
One advantage of a swing arm type pick mechanism is the self-adjusting relationship between the pick load and the pick force. The swing arm pivot above and behind the pick roller allows the pick force exerted by the pick roller to increase automatically as the pick load (the top sheet's resistance to movement) increases. For example, heavier media increases the top sheet's resistance to movement. Heavy media, therefore, creates a higher pick load. Consequently, a larger pick force is necessary to move the top sheet off the stack. As the pick load increases, however, the force of the pick roller against the unmoving top sheet drives the swing arm down and presses the pick roller harder against the stack, increasing the normal force, to automatically increase the pick force. This effect becomes more pronounced as the stack gets smaller. Unfortunately, as the pick roller presses harder against the stack, the friction between the sheets increases which, in turn, increases the pick load. The increased pick load negates some of the self-adjusting effect of the swing arm pick mechanism.
The present invention was developed in an effort to reduce the pick load in a swing arm type mechanism, particularly for heavy, stiff and other higher friction print media.